1. Field of Invention
This invention relates to radar, specifically to improved detection of low observable targets in simultaneous clutter and jamming interference.
2. Description of Prior Art
Radar detection of aircraft and surface targets has historically been limited by surface clutter and thermal noise. Additionally, for military application, targets have been increasingly protected by onboard or stand-off support deception and noise type jamming.
The ability of any radar to recognize a desired target from multiple echo returns, deceptive interference and thermal noise continues to a major radar design challenge. The problem is very difficult when the desired target echo is buried in surface clutter and/or deception jamming.
The modern jamming approach to solution has enhanced resolution in the three information domains: angle (direction), time (range), frequency (Doppler). Basis has been application of fundamental mathematical Fourier analysis and the more recent estimation principles based on information theory. Matched filter-correlation receivers optimize signal to thermal noise. Large time-bandwidth product waveforms increase resolution in Doppler-range.
But, the enhanced resolution has still not removed ambiguities. Sidelobe ambiguities generated by interfering echo waveforms tend to cover desired targets, when desired target echos are weak and in close proximity.
FIGS. 1-4 illustrate Fourier transform relationships for prior art classes of radar waveforms: "Fundamental", "Burst", "Intrapulse".
The Fourier transform from time domain into frequency domain for each waveform illustrates the resolution and ambiguity in frequency (Doppler). The Fourier transform from the first transform squared into the autocorrelation waveform illustrates the resolution and ambiguity in time (range).
The "Fundamental" waveform class is illustrated by a single rectangular pulse. It is ambiguous in frequency, with the first ambiguous sidelobe down 13 dB from the peak. Frequency resolution is the reciprocal of pulse duration. Its autocorrelation waveform is unambiguous in time. Time resolution is proportional to pulse duration.
The "Fundamental" waveform is the classic waveform of early radars. The transform from time to frequency also applies to antenna pattern generation, where time domain represents antenna aperture illumination. The transform represents antenna pattern shape (with ambiguous angle sidelobes). Shaping of the time domain envelope (or aperture illumination) reduces sidelobe amplitudes, but, at the expense of wider main lobe width and reduced time (or angle) resolution.
The "burst" waveform class is illustrated with a high duty cycle sequence of three pulses. "Burst" is the form of waveform used with Pulse Doppler radars, for coherent integration Doppler detection of moving targets. It is ambiguous in both frequency (Doppler) and time (range). Resolution in frequency is increased over the "Fundamental" single pulse, but not in time (range). Resolution in Doppler is the reciprocal of signal duration; typical kilohertz Doppler resolution requires signal integration time of milleseconds, with coherent processing.
The "Intrapulse" waveform class represents the more recent Ambiguity Function developed radar waveform advances. These provide both increased resolution and reduced ambiguity in simultaneous frequency and time. These also permit transmission of long duration pulses, with high energy. In conjunction with matched filter receiver processing, "Intrapulse" enhances Signal to Noise while at the same time improves target resolution in the presence of unwanted interference.
The linear FM "chirp" Intrapulse waveform shown is very ambiguous in frequency. Autocorrelation (time) resolution and processing gain are proportional to transmitted waveform time-bandwidth product. But, it is also ambiguous in time due to autocorrelation sidelobes. These sidelobe magnitudes may be suppressed, at the expense of reduced resolution, by tapering the auto-correlation receiver dechirp envelope (weighting). The analogous process supresses antenna pattern sidelobes with tapered aperture illumination.
The 13 bit Barker phase code further enhances combined energy, resolution and ambiguity.
However, all of the above waveforms suffer from a number of disadvantages:
(a) Finite level ambiguity frequency and/or time sidelobes. PA1 (b) Reduced resolution with sidelobe suppression. PA1 (c) Complex modulations to provide both high energy, long duration signals, with high resolution and suppressed sidelobe ambiguities. PA1 (d) Limited detection and identification of desired low visibility targets in simultaneous clutter/jamming interference. PA1 (a) to eliminate ambiguities in range. PA1 (b) to eliminate susceptabilities to jamming deception in range. PA1 (c) to provide high probability, single pulse detection and identification of desired low visibility targets in simultaneous clutter/jamming interference. PA1 (d) to provide high energy content transmissions for matched filter/correlation receiver detection of low visibility targets with high signal to thermal noise. PA1 (e) to provide unambiguous high resolution in range with a single, high energy, long duration pulse. PA1 (f) to provide implementation using simple, conventional, non coherent, pulsed magnetron transmitters.